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From: kimberln@crl.com (J. Kimberlin)
Newsgroups: rec.crafts.metalworking
Subject: Blackening Aluminum
Date: 12 Aug 1995 20:11:13 -0700

There have been several postings about blackening aluminum recently.  I 
wrote an article for *Live Steam* magazine back about 1977 on the subject 
and have modified it a little since then.  I do use it myself and thought 
that it would be of interest to the newsgroup.  So:

			BLACKENING ALUMINUM
				 by
			  JEROME KIMBERLIN

	There are several ways to color aluminum black and among them
 are black anodizing and paint.  You could rub dirt into the aluminum
 surface, I suppose, but of all the methods, I think chemical coloring
 is the superior method.  It is certainly cheaper, faster, and home use
 allows the model engineer greater flexibility in the timing of his
 decoration of models in progress.

	Surface preparation of parts to be colored black is all
 important as any irregularities are not covered by this finish.  Paint
 does build up and fill in scratches and other voids.  Castings,
 however, should look like castings if the prototype used castings, so
 surface finish is always adjustable to the builders idea.  The point
 here is to emphasize that this blackening technique will not cover up
 mistakes.

	You will need three chemicals.  These are: Nitric Acid, Copper
 Nitrate, and Potassium Permanganate.  You will also need some good
 quality water - either distilled or deionized.  I will give the
 dimensions of the mixture in both metric and English units so that
 both types of measures are accommodated:

	Take:    water         3 quarts          750ml
	Add      Acid          1/2 oz              5ml
	Add      Copper        3 oz               25gm
	Add      Permanganate  1 oz               10gm
	Add      Water to make 1 gal              1 liter

	Obviously you will have to make up more or less solution to
 fill the container you will use to color aluminum parts and the parts
 to be colored should be completely covered by the solution.  You
 should use a glass or plastic container.  A metal container will
 poison the solution prematurely.

	At 75 degrees F (24 C) temperature, the blackening process will
 take about 15 minutes using a fresh solution.  If it takes longer it
 means the solution is deficient in one of the components.  Usually,
 copper nitrate and nitric acid need be added.

	Aluminum is a strange metal to most of us.  While we cannot see
 it, the surface of a newly machined or cleaned piece of aluminum
 combines with oxygen in the air to form a self protecting coating of
 aluminum oxide.  This happens within minutes.  If this surface
 continues to grow (get thicker) the blackening solution described here
 will not work satisfactorily.  Thus, the piece to be colored should be
 cleaned just before immersing into the coloring solution.  In my
 experience, glass bead blasting is a superior way to clean the
 aluminum surface and the choice of bead size determines surface
 finish.  Once the bead blasting has been accomplished, the beads can
 be washed off with hot water and the aluminum piece immersed in the
 blackening solution.  I recommend that the time between blasting
 (cleaning) and immersion in the blackening solution be less than two
 hours.  I once waited five hours and was disappointed in the results.
 Once the blackening process has been completed, wash off the workpiece
 with tap water, drain and spray with WD-40 or other water displacing
 oil.

	There are a number of ways to clean aluminum satisfactorily.
 It is possible to simply sand the surface clean, or scrub it clean
 with an abrasive.  One can also chem clean aluminum by degreasing the
 workpiece then dipping it into lye (Draino, for instance) for a few
 minutes or seconds as required, then rinsing.  The shape of the
 workpiece and the model engineer's facilities often dictate what
 method of surface preparation will be used.

	Model engineers wishing to use this solution to blacken
 aluminum castings or other parts should be aware that the chemical
 components may be hazardous.  While the solution itself is not
 particularly dangerous it can make your hands purple, so use rubber or
 plastic gloves.  Potassium Permanganate is classified as an oxidizer
 even though dilute solutions of it are used throughout the world to
 sterilize vegetables used in salads, etc.  Concen trated nitric acid
 is just plain bad.  The technique for using it is to pour out a little
 in a glass container and then use an eye dropper to transfer the
 liquid to a measuring container when the volume wanted is small, such
 as that described here.  Nitric acid also turns your hands yellow,
 hurts, and removes fingerprints.  A good way to avoid eye damage is to
 wear a face shield such as the one you should be wearing when working
 in front of your grinder.

Newsgroups: rec.crafts.metalworking
Subject: Re: diy chrome plating? (faq?)
From: kimberln@crl.com (J. Kimberlin)
Date: 29 Sep 1995 17:00:28 -0700

In article <wwebster-2909950737310001@47.116.15.87>,
Bill Webster <wwebster@bnr.ca> wrote:
>In article <44ffr6$mu6@crl7.crl.com>, kimberln@crl.com (J. Kimberlin) wrote:
>
>> In article <44eodm$236@nntp.nta.no>,
>> Ole-Hjalmar Kristensen TF.E/DELAB <ohk@hal.nta.no> wrote:
>
>> >Brownells sells kits for doing electroless nickel plating, maybe they could
>> >direct you to their supplier.
>> 
>> I wrote an article on Electroless Nickel plating back in 76 or 77.  Jim 
>> has it at Plains as part of the FAQ or addendum there-to, or will have 
>> eventually.  Electroless Nickel is rather easy to do, once you are set up 
>> and have done it a time or two.  In short, you need some chemicals, a 
>> glass vessel big enough to hold your stuff, and a hotplate good enough to 
>> get up to about 190 deg F and hold it plus/minus 2 deg for 20 minutes or 
>> so.  You can put electroless nickel on just about anything, even glass 
>> and plastic.  It will come out looking just like what you put in, smooth, 
>> scratches, dull, polished, whatever.  I used to use it with a bead 
>> blasted matte finish - very nice finish.
>> 
>> JerryK
>
>Can electroless Nickel plating also be used on aluminum?  If so, how does
>the aluminum get prepared to avoid oxydation?  I'd like to try this.


I've never tried it on aluminum, but generally aluminum oxidizes in 
minutes or seconds after cleaning.  In the past I have cleaned aluminum 
by bead blasting and instantly submurged it in oxygen free water.  You 
can get oxygen free water two ways, put in an oxygen getter such as 
hydrazine or sodium sulfite or simply bubble nitrogen through it for awhile.

My article on Aluminum blackening using permanganate solution also 
requires very clean aluminum for best results.  I know the problem and it 
has tested my patience many times!

JerryK

 
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